This invention provides a sorbing method for recovering carboxylic acid from aqueous solutions at pHs above the pK.sub.a of the carboxylic acid(s), i.e., under conditions where the carboxylic acid is present in solution largely as a carboxylate salt, and thereafter regenerating the sorbent. Carboxylic acids are important articles of commerce and are among the most attractive chemical products for manufacture from biomass by fermentation. Adipic acid is a large-scale raw material for manufacture of nylon. Citric, acetic, propionic, butyric, lactic, succinic, malic and fumaric acids are other important current and potential products. Most fermentations producing carboxylic acids work best (or work only at all) at solution pHs above the pK.sub.a of the acid, and this invention provides an effective method of recovery and regeneration under these conditions.
Extraction has been a well-established sorbent method for recovery of carboxylic acids. It has also been recognized that there are solid phase sorbents which will take up carboxylic acids effectively. For acids with low volatilities (e.g., dicarboxylic acids and hydroxycarboxylic acids) methods for regenerating extractants and solid sorbents have been limited. Heretofore, the only workable method of using extractants and sorbents strong enough to extract a carboxylic acid from a solution at pH above the pK.sub.a has been back-extraction with an aqueous solution of an inorganic base, such as calcium hydroxide. Recovery of the carboxylic acid then requires addition of a strong acid, such as sulfuric acid, to the aqueous back-extract so as to precipitate calcium sulfate and yield an aqueous solution of carboxylic acid. The result is that both sulfuric acid and calcium hydroxide are consumed and a waste calcium sulfate salt stream is produced.
Electrodialysis with "water-splitting" membranes is also claimed to have the capability of recovering free carboxylic acid from aqueous carboxylate solutions. This approach is still developmental, and no industrial installation is known yet to have been built.
As can be seen from this description of background, various methods used heretofore to recover carboxylic acids have presented limitations and thus offer opportunities for improvement. It is accordingly a general object of the invention to provide an efficient process for the recovery of carboxylic acids from aqueous solutions at pHs close to or above their pK.sub.a s which neither consumes large amounts of chemicals nor generates substantial waste chemicals.